This invention relates to a method for improving energy recovery in various industrial stripping processes wherein a gaseous component is removed from a liquid by effecting its volatilization and then carrying it away by contact with a stream of hot, condensable vapor. More particularly, the invention is concerned with stripping absorbed gases from liquid absorbents used in various gas-purification processes and with removing high-vapor-pressure contaminants from process condensate and other waste water streams. These are common, energy-consuming unit operations in industrial chemistry.
Stripping operations such as these involve the removal of relatively noncondensable, high-vapor-pressure gaseous components from liquid mixtures. In the case of aqueous solutions, steam is the preferred stripping vapor, and it may be supplied either by reboiling the aqueous solution being stripped and vaporizing a portion of it or by injecting live steam near the base of the stripping column. Organic liquids can be stripped of gases in a similar fashion. Stripping vapor and liquid may be contacted, usually in a countercurrent fashion, in a packed column, a plate column, or other type of two-phase contactor as described by Ludwig in Volume 2 of Applied Process Design for Chemical and Petrochemical Plants (1964).
Stripping vapor serves three purposes in a stripping operation. First, it provides the energy required to heat the liquid (thereby increasing the tendency of dissolved gases to desorb). Secondly, it supplies the heat effect associated with gas desorption. This heat effect is typically small for a physically sorbed gas where only its heat of solution is involved, but the heat effect can be much larger when reversible chemical reactions take place in sorption. Thirdly, stripping vapor serves as a diluent for desorbed gases, thereby increasing the driving force for desorption and sweeping desorbed gases out of the stripping vessel.
A condensable vapor is preferred as the stripping agent because the vapor is readily separated from the stripped gas by condensation in an overhead condenser. Additionally, a vapor possesses the advantages of being easily generated by reboiling the stripped solution, and it can carry large quantities of energy in the form of its latent heat of vaporization.
Unfortunately, separation of the stripping vapor from the stripped gas in the gas/vapor overhead mixture is a very wasteful process, involving as it does condensation of the vapor and attendant loss of its latent heat to the condenser cooling medium, usually cooling water. Because the latent heat of the mixed vapor is lost, external energy must be supplied in order to generate fresh stripping vapor. As a result, the unit operation of stripping a gas from a liquid is generally a very energy-intensive one.
Stripping operations arise in a large number of chemical processing situations. For example, the most important gas-purification technique is undoubtedly gas absorption, and regeneration of the rich absorbent is generally accomplished in a stripping step (sometimes accompanied by a flashing operation). For example, hydrogen sulfide and carbon dioxide are often removed from sour natural gas, coal gas and chemical process/refinery gas streams by absorption in aqueous solutions of reversibly reactive bases. Examples of H.sub.2 S and C0.sub.2 removal processes employing aqueous absorbents include the mono- and diethanolamine processes, the Sulfinol process, the diglycolamine process, and the carbonate processes including the Benfield, Catacarb, and Giammarco-Vetrocoke processes. Each of these absorbents is regenerated in a steam stripping operation, either by injection of live steam into the stripper or by reboiling the lean stripper bottoms liquor. In addition to the above processes involving chemically reactive absorbents, there exist other processes for H.sub.2 S and C0.sub.2 removal wherein the absorbent liquid is merely a physical solvent for the acid gases. The Rectisol and Selexol processes provide two examples. Again, solvent regeneration is accomplished in these processes by steam stripping. Ammonia, sulfur dioxide, carbon monoxide, and still other gases can be removed by absorption/desorption process technology as described by Kohl and Riesenfeld in Gas Purification, 3rd edition (1979).
Steam stripping operations also arise in the context of removing high-vapor-pressure contaminants from aqueous process waste streams. For example, sour water and other chemical process and refinery condensate streams frequently contain high concentrations of H.sub.2 S and NH.sub.3 that must be reduced prior to disposal or reuse of the water. Holiday describes such processes in Chemical Engineering 90(1982)118. Separation of these gases is frequently accomplished by steam stripping the contaminated stream. In other processes, for example, in the production of polymeric resins, process-water streams may be contaminated with highly volatile (albeit condensable) solvents such as methylene chloride. Steam stripping is frequently employed for removal of such volatile solvents from waste water streams.
A number of methods have been proposed for recovery of the energy and in particular the latent heat contained in the gas vapor mixture produced in a stripping operation. For example, instead of losing the heat of condensation to cooling water in a water-cooled condenser, it is known that stripping efficiency can be improved by transferring the energy contained in the gas/vapor overhead mixture to the stripper feed stream in a heat exchanger/condenser designed to preheat the column feed. Alternatively, the overhead mixture can be condensed against some other process stream which it is necessary to heat to a temperature somewhat below the condensing range of the gas/vapor mixture from the stripper. Unfortunately, there are usually few places for economical recovery of energy at the relatively low temperatures associated with condensation of the gas/vapor overhead mixture.
A more generally applicable method for improving the energy efficiency of stripping operations is vapor compression as discussed by King in Separation Processes, pages 695-699, 2nd edition (1980) and in the Holiday reference cited above. In this technique, the overhead vapor from the stripping column is mechanically compressed and subsequently condensed with recovery of its latent heat, usually in an indirect heat exchanger which serves to reboil the stripped liquid. Although vapor compression works well in the absence of noncondensable gases (e.g., in distillation) or in stripping operations in which the concentration of noncondensables in the overhead mixture is small, the technique is not very efficient when large concentrations of stripped gas are present in the overhead mixture. In such cases, the method can be impractical due to the large energy requirement associated with compressing all of the stripped gas along with the stripping vapor. Moreover, the presence of large quantities of noncondensable gases in the mixture limits the fraction of the vapor which can be condensed and recovered at a particular set of operating conditions. Finally, noncondensable gases have the undesirable effect of blanketing heat transfer surfaces and reducing rates of heat exchange in the condenser/reboiler. As a result, applications of vapor compression schemes to stripping operations are limited.
Another heat recovery technique known in the art is the use of heat pumps to extract energy from the gas/vapor overhead mixture at the condensing temperature and to return it to the process at the higher temperature associated with solution reboiling. Heat pump schemes are similar in concept to vapor compression, with the exception that an external working fluid is employed in the former. They are generally subject to the same limitations inherent in vapor compression when applied to stripping processes.
Semipermeable media have been used in the past for the recovery of vapors from mixtures with gases. For example, Booth, in U.S. Pat. No. 3,420,069, describes a condenser-separator in which a heat exchanger constructed from porous sintered metal tubes is used to remove condensed and entrained liquids from gas streams, although without significant heat recovery. Ketteringham and Leffler, in U.S. Pat. No. 3,511,031, describe a similar means for dehumidifying air in an enclosed space such as the cabin of a spacecraft by condensing water vapor in pores of microporous membranes, again without heat recovery. Finally, Salemme, in U.S. Pat. No. 3,735,559, discloses a process for removing water from a moist air stream which utilizes a permselective membrane for water vapor transport. However, none of these patents is concerned with the stripping of gases from liquids, and none discloses significant recovery of the latent heat of the vapor for reuse.
It is further known in the art that microporous membranes may be used in distillation processes. The membrane separates distilland from distillate in these schemes, and a temperature gradient is generally maintained across it. The temperature gradient effects evaporation of the more volatile component from the distilland mixture on one side of the membrane, and it effects condensation of distillate on the other. For example, distillation methods and apparatus employing microporous membranes are disclosed by Cheng in U.S. Pat. No. 4,265,713 and by Rodgers in U.S. Pat. Nos. 3,477,917, 3,650,905, 3,661,721, 3,765,891, and 3,896,004. Composite membranes comprised of hydrophobic and hydrophilic or of lyophobic and lyophilic regions have also been described for this purpose by Cheng in U.S. Pat. Nos. 4,265,713 and 4,316,772, respectively. Pampel, in U.S. Pat. No. 4,301,111, further discloses the use of a microporous membrane with an air pressure gradient maintained across it in order to effect distillation of a liquid mixture. However, none of the above patents is concerned with the stripping of gases from the liquid distilland, and none discloses significant recovery of the latent heat of the vapor for reuse. Moreover, the microporous membranes employed in these cases are not selective for the permeation of vapor in preference to gas.
Guarino, in U.S. Pat. No. 3,540,986, discloses the use of a membrane in a distillation/condensation apparatus wherein heat is recovered from the distilled vapor following vapor compression. However, the membrane employed is microporous and serves only to separate distilland liquid from distillate vapor (there being no noncondensable gas present and the membrane having no vapor/gas selectivity).
To summarize, known industrial processes for the removal of noncondensable gases from liquid absorbents and process waste water streams frequently involve contacting the liquid with a relatively hot stripping vapor that carries the desorbed gases from the stripper and supplies the energy required to heat the liquid and to desorb the gas. The mixture of stripped gases and stripping vapor that leaves the top of the column is frequently routed to an overhead condenser or heat exchanger which condenses the vapor and thereby separates it from the stripped gases. Recovery of the energy in the stripper overhead mixture is a longstanding and important problem that remains to be solved in an economical and practical manner.
It is therefore an object of the present invention to provide a method for recovering a significant fraction of the sensible and latent heat present in the gas/vapor overhead mixture from a stripping column in order to permit reuse of that energy in the stripping process and thereby to improve the energy efficiency of stripping to an extent and in a manner not previously contemplated.
This object is accomplished by the present invention, which is summarized and described below.